<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<title></title>
	<style type="text/css">
		*{
			margin: 0;
			padding: 0;
		}
		.box{
/*			cala 帮我们计算相对单位和绝对单位*/
/*			写个盒子 左外边距 30px 右外边距 20px*/
			width: calc(100% - 50px);
			height: 100px;
			margin-left:30px ;
			margin-right: 20px;
			background-color: red;
		}
	</style>
</head>
<body>
	<div class="box"></div>
</body>
</html>